1. Field of the Invention
The preferred embodiments of the present invention are directed to the drilling of oil and gas wells. More particularly, the invention relates to operations that are engaged in while a drill or tool string is downhole. In one aspect, the present invention relates to measuring-while-drilling (MWD) and logging-while-drilling (LWD) systems and other systems and methods for drilling wellbores and simultaneously measuring and recording certain characteristics of the well, particularly when evaluating subsurface zones of interest while these zones are being intersected by the drill string.
2. Background of the Invention
During the drilling and completion of oil and gas wells, it is often necessary to engage in ancillary operations, such as monitoring the operability of equipment used during the drilling process or evaluating the production capabilities of formations intersected by the wellbore. For example, after a well or well interval has been drilled, zones of interest are often tested to determine various formation properties such as permeability, fluid type, fluid quality, formation pressure, and formation pressure gradient. These tests are performed in order to determine whether commercial exploitation of the intersected formations is viable.
In the past, wireline formation testers (WFT) and drill stem testing (DST) were most commonly used to perform these tests. DST is one conventional method of formation testing. The basic work stem test tool consists of a packer or packers, valves or ports that may be opened and closed from the surface, and two or more pressure-recording devices. The tool is lowered on a work string to the zone to be tested. The packer or packers are set, and drilling fluid is evacuated to isolate the zone from the drilling fluid column. The valves or ports are then opened to allow flow from the formation to the tool for testing while the recorders chart static pressures. A sampling chamber traps clean formation fluids at the end of the test. WFT's generally employ the same testing techniques but use a wireline to lower the test tool into the well bore after the drill string has been retrieved from the well bore. The wireline tool typically uses packers also, although the packers are placed closer together, compared to drill pipe conveyed testers, for more efficient formation testing. In some cases, packers are not used. In those instances, the testing tool is brought into contact with the intersected formation and testing is done without zonal isolation. Although WFT's were employed before DST, WFT's continue to be used for their efficiency and cost-effectiveness in certain situations.
As important as these tools are to production and reservoir engineering, their use can be limited by numerous factors. The amount of time and money required to run these tools downhole can be significant, especially with today's increasingly costly drilling rigs. First, the drill string with the drill bit must be retracted from the wellbore. Then, a separate work string containing the testing equipment, or, if wireline services are used, the wireline tool string, must be lowered into the well to conduct secondary operations. Interrupting the drilling process to perform formation testing can add significant amounts of time to a drilling program, which can be prohibitively expensive with today's drilling rigs. Thus, by interrupting the drilling process, operational costs can become high even though the cost of the DST or WFT itself may be reasonable.
DST and WFT pose additional risks to the borehole, such as tool sticking or formation damage. Specific to WFT are the difficulties of running wireline services in highly deviated and extended reach wells. WFT's also do not have flowbores for the flow of drilling mud, nor are they designed to withstand drilling loads such as torque and weight on bit.
Further, the measurement accuracy of drill stem tests and, especially, of wireline formation tests can be affected by mud invasion and filter cake buildup because significant amounts of time must pass before a DST or WFT may engage the formation. Mud invasion occurs when formation fluids are displaced by drilling mud or mud filtrate. Because the drilling mud ingress begins at the wellbore surface, it is most prevalent there and generally decreases further into the formation. However, the prevalence of the mud invasion at the wellbore surface creates a “skin” or “mudcake,” and a “skin effect” may occur because formation testers can only extend relatively short distances into the formation, thereby distorting the representative sample of formation fluids. When invasion occurs, it may become impossible to obtain a representative sample of formation fluids or, at a minimum, the duration of the sampling period must be increased to first remove the drilling fluid and then obtain a representative sample of formation fluids.
Similarly, as drilling fluid with its suspended solids is pumped downhole, the fluid engages the walls or surface of the wellbore and, in a fluid permeable zone, leaves suspended solids on the wellbore surface. If a large amount of solids attach themselves to the well bore surface, a filter cake buildup occurs. The filter cakes act as a region of reduced permeability adjacent to the wellbore. Thus, once filter cakes have formed, the accuracy of reservoir pressure measurements decreases, affecting the calculations for permeability and produceability of the formation.
Consequently, it is of considerable economic importance for tests such as those described hereinabove to be performed as soon as possible after the formation has been intersected by the wellbore, and without interrupting the drilling process. Mud invasion and filter cake buildup increase with time after penetration of the formation, thereby reducing the accuracy of formation test results. Therefore, early evaluation of the potential for profitable recovery of the fluid contained therein is very desirable. For example, such early evaluation enables completion operations to be planned more efficiently. In addition, it has been found that more accurate and useful information can be obtained if testing occurs as soon as possible after penetration of the formation.
In the late 1970's, MWD/LWD technology was born to address the needs of the industry. MWD/LWD technology became mature about a decade later, and eventually incorporated the concept of formation testing. Where early formation evaluation is actually accomplished during drilling operations within the well, the drilling operations may also be more efficiently performed, since results of the early evaluation may then be used to adjust parameters of the drilling operations without interrupting the drilling process. In this respect, it is known in the art to integrate certain formation testing equipment with a drill string so that, as the wellbore is being drilled, and without removing the drill string from the wellbore, formations intersected by the wellbore may be periodically tested.
In typical prior art formation testing equipment suitable for integration with a drill string during drilling operations, various devices or systems are provided for isolating a formation from the remainder of the wellbore, drawing fluid from the formation, and measuring physical properties of the fluid and the formation. Unfortunately, due to the constraints imposed by the necessity of integrating testing equipment with the drill string, problems do exist when using typical prior art formation testing equipment.
For example, formation testing equipment is subject to harsh conditions in the wellbore during the drilling process that can damage and degrade the formation testing equipment before and during the testing process. These harsh conditions include vibration and torque from the drill bit, exposure to drilling mud, drilled cuttings, and formation fluids, hydraulic forces of the circulating drilling mud, and scraping of the formation testing equipment against the sides of the wellbore. Sensitive electronics and sensors must be robust enough to withstand the pressures and temperatures, and especially the extreme vibration and shock conditions of the drilling environment, yet maintain accuracy, repeatability, and reliability. Therefore, it is highly desirable for while drilling formation tester systems to be appropriately ruggedized for downhole conditions while maintaining the necessary precision for useful formation measurements. Conventional drilling formation testing tools are not rugged enough for harsh drilling environments, and have not been able to achieve the precision and durability required for efficient formation testing.
In one aspect of formation testing, the formation testing apparatus may include a probe assembly for engaging the borehole wall and acquiring formation fluid samples. The probe assembly may include an isolation pad to engage the borehole wall, or any mudcake accumulated thereon. The isolation pad seals against the mudcake and around a hollow probe, which places an internal cavity in fluid communication with the formation. This creates a fluid pathway that allows formation fluid to flow between the formation and the formation tester while isolated from the wellbore fluid.
In order to acquire a useful sample, the probe must stay isolated from the relative high pressure of the wellbore fluid. Therefore, the integrity of the seal that is formed by the isolation pad is critical to the performance of the tool. If the wellbore fluid is allowed to leak into the collected formation fluids, a non-representative sample will be obtained and the test will have to be repeated.
Examples of isolation pads and probes used in wireline formation testers include Halliburton's DT, SFTT, SFT4, and RDT. Isolation pads that are used with wireline formation testers are generally simple rubber pads affixed to the end of the extending sample probe. The rubber is normally affixed to a metallic plate that provides support to the rubber as well as a connection to the probe. These rubber pads are often molded to fit within the specific diameter hole in which they will be operating.
While conventional rubber pads are reasonably effective in some wireline operations, when a formation tester is used in a MWD or LWD application, they have not performed as desired. Failure of conventional rubber pads has also been a concern in wireline applications that may require the performance of a large number of formation pressure tests during a single run into the wellbore, especially in wells having particularly harsh operating conditions. In a MWD or LWD environment, the formation tester is integrated into the drill string and is thus subjected to the harsh downhole environment for a much longer period than in a wireline testing application. In addition, during drilling, the formation tester is constantly rotated with the drill string and may contact the side of the wellbore and damage any exposed isolator pads. The pads may also be damaged during drilling by the drill cuttings that are being circulated through the wellbore by the drilling fluid.
Therefore, in addition to ruggedizing the overall apparatus for use as a while drilling, MWD-based formation tester, there remains a need in the art to develop an isolation pad that provides reliable sealing performance with an increased durability and resistance to damage. Furthermore, in addition to these characteristics, the industry would welcome a field replaceable pad for use in the while drilling formation tester.